1. Field of the Invention
The present invention relates to a bearing bush for a seat reclining device and the seat reclining device. More particularly the present invention relates to a bearing bush for a reclining device suitable for a car seat and the seat reclining device having the bearing bush.
2. Description of the Related Art
To adjust an open angle of a hinge connecting a seating surface of a seat and a backrest thereof to each other, in a conventional seat reclining device, a method of adjusting the tilt angle of the backrest by using a differential transmission mechanism having gears is adopted. The differential transmission mechanism of the seat reclining device using the gears is described below with reference to FIG. 2 which is a cut-out sectional view showing a part of main parts of the conventional reclining device. In the description of an internal gear 1 of the differential transmission mechanism, FIG. 1 which is a cut-out sectional view showing a part of main parts of the seat reclining device of one embodiment of the present invention is used.
The conventional seat reclining device has the gear-provided differential transmission mechanism provided for the hinge connecting the seating surface of the seat and the backrest thereof to each other. The differential transmission mechanism has the internal gear 1 (see FIG. 1), an external gear 2, having a smaller number of teeth than the internal gear 1, which engages the internal gears 1, and a ring-shaped bearing bush 10 fitted in a shaft hole of the external gear 2. A pair of wedge-shaped pieces 16, 16′ is disposed in a circular arc-shaped gap formed between the ring-shaped bearing bush 10 and a shaft 4 of the internal gear 1 (see FIG. 1) as well as a bearing bush 11 of the shaft 4 with tapered ends 16a, 16a′ of the wedge-shaped pieces 16, 16′ opposed to each other. The differential transmission mechanism further includes a compression coil spring 7 for imparting an elastic force to the wedge-shaped pieces 16, 16′ in a direction in which the wedge-shaped pieces 16, 16′ depart from each other. The bearing bush 11 provided on the shaft 4 of the internal gear 1 (see FIG. 1) and a projection 11a of the bearing bush 11 are operated. Thereby the wedge-shaped pieces 16, 16′ move inside the circular arc-shaped gap with the wedge-shaped pieces 16, 16′ in frictional sliding contact with the bearing bush 10. As a result, the external gear 2 separates from the internal gear 1 (see FIG. 1). Thereby the tilt angle of the backrest is adjustable.
The above-described differential transmission mechanism is called a Taumeru mechanism. The seat reclining device of the car using the differential transmission mechanism has the bearing bush 10 disposed inside the external gear 2, as described above. The wedge-shaped pieces 16, 16′ make frictional sliding contact with the bearing bush 10 (see patent documents 1 and 2). In any conventional seat reclining device of the car having the construction in which the wedge-shaped pieces 16, 16′ make frictional sliding contact with the bearing bush 10, it is necessary to insert the bearing bush 10 into the external gear 2 by press fit to smoothen the operation of the wedge-shaped pieces 16, 16′. The sliding surfaces of the bearing bushes 10, 11 are made of resin excellent in the sliding property thereof. For example, as shown in FIG. 3, a multilayered bearing (bearing bush) is known. The multilayered bearing (bearing bush) has a layered structure composed of a back metal 10a, a porous sintered layer 10b, consisting of a sintered molding, which is layered on the back metal 10a, and a resin layer 10c, consisting of a slidable resin composition, a part of which is filled in pores of the porous sintered layer 10b. 
As the multilayered bearing for the seat reclining device, a multilayered bearing having polytetrafluoroethylene (hereinafter referred to as PTFE) resin containing lead or lead oxide as a solid lubricant for a high surface pressure in its resin layer is conventionally used. But owing to the prohibition of the use of lead oxides caused by the issue of the ELV directive, the RoHS directive, and the like, the use of a multilayered bearing not containing the lead is demanded. For example, as the multilayered bearing not containing the lead, the multilayered bearing composed of the resin, containing the PTFE resin as its main component, to which at least the granular inorganic filler whose particles have an average diameter of 1 to 50 μm is added is known (see patent document 3).
But when the multilayered bearing, disclosed in the patent document 3, composed of the resin, containing the PTFE resin as its main component, to which at least the granular inorganic filler whose particles have the average diameter of 1 to 50 μm is added is used for the seat reclining device of a car seat, the multilayered bearing has sliding property inferior to that of the lead-containing multilayered bearing at a high surface pressure. In the seat reclining device disclosed in the patent document 3, the resin layer of the multilayered bearing and the wedge-shaped piece repeatedly frictionally contact each other. Therefore the seat reclining device disclosed in the patent document 3 has a problem that the resin layer of the multilayered bearing peels off the adjacent layer thereof and wears. A one-size larger bearing bush is adopted to solve the problem of the wear of the resin layer of the multilayered bearing. Alternatively a proposal of forming a sliding material having a low friction coefficient on the frictional sliding-contact surface of the wedge-shaped piece has been made (see patent document 4).    Patent document 1: Japanese Patent Application Laid-Open No. 03-237904    Patent document 2: Japanese Patent Application Laid-Open No. 2004-033401    Patent document 3: Japanese Patent Application Laid-Open No. 2002-327750    Patent document 4: Japanese Patent Application Laid-Open No. 2008-006265